Hybrid vehicles and electric vehicles have recently attracted attention as environmentally friendly vehicles. A hybrid vehicle is a vehicle including, in addition to a conventional engine, a motor driven by a DC power supply via an inverter, as a motive power source. Namely, the hybrid vehicle obtains a motive power source by driving the engine and also obtains a motive power source by converting a DC voltage from the DC power supply to an AC voltage through an inverter and revolving the motor with the resultant AC voltage.
Meanwhile, an electric vehicle is a vehicle including a motor driven by a DC power supply via an inverter as a motive power source.
In many cases, an inverter includes a semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field Effect Transistor). A breakdown voltage (hereinafter simply referred to as a “withstand voltage”) of the IGBT or the MOSFET is lower as a temperature around the same or a junction temperature is lower. Therefore, an input voltage of the inverter should appropriately be controlled in accordance with an ambient temperature of the inverter.
Japanese Patent Laying-Open No. 2004-166341 discloses a voltage conversion device for converting a voltage from a power supply such that operation performance of an inverter is prevented from lowering when an ambient temperature lowers and a motor counterelectromotive force voltage is higher than an inverter withstand voltage. This voltage conversion device includes a voltage converter for varying a voltage level of a voltage input from a power supply and supplying an output voltage to an electric load, and a drive circuit for driving the voltage converter such that a temperature of the electric load is raised. Specifically, as a temperature of cooling water for the inverter is significantly lower than a reference temperature, a boost ratio of a boost converter (voltage converter) is set higher. As loss of the inverter (electric load) is thus further increased, the temperature of the inverter is raised.
In the inverter including the semiconductor switching element, a surge voltage is produced due to parasitic inductance of a line while the semiconductor switching element is turned off. Here, the surge voltage refers to a voltage increase from the input voltage of the inverter to the counterelectromotive force voltage produced by the parasitic inductance. In order for the counterelectromotive force voltage not to exceed the withstand voltage of the semiconductor switching element, the input voltage of the inverter should be determined in consideration of the voltage increase by the surge voltage.
When the temperature of the semiconductor switching element is low, however, the withstand voltage of the semiconductor switching element is lowered. When the input voltage of the inverter is constant in spite of the low withstand voltage of the semiconductor switching element, a difference between the withstand voltage and the input voltage, that is, a tolerable amount of the surge voltage, becomes smaller. Accordingly, as the inverter is driven while the temperature of the semiconductor switching element is low, the semiconductor switching element may adversely be affected. Japanese Patent Laying-Open No. 2004-166341 does not particularly mention control of the voltage converter when the temperature of the electric load is not yet sufficiently high.